Traditionally, mammalian cells, as well as bacterial cells, are primarily cultured as suspension cultures in bioreactors, which are also, called fermenters. The environmental conditions can be precisely controlled in such vessels. A stirring means moves the culture medium in the interior of the reactor and thus provides for a homogeneous distribution of the cells.
The supply of nutrients to the cells and the removal of waste materials take place, in the case of liquid suspension cultures in bioreactors, according to one of the following processes:
In a batch operation, the reactor is operated discontinuously. At the beginning of a batch, the culture medium usually contains a medium with the necessary nutrients, for example glucose, vitamins, amino acids and minerals. During fermentation, these are consumed so that the medium becomes more and more deprived in nutrients. At the same time, the concentration of waste products increases which finally results in a prevention of the cell growth. The viable cell density achieves a maximum value and thereafter decreases again. Consequently, the culturing is normally discontinued when the maximum cell density is reached or a minimum cell viability is reached. The content of the reactor is then passed on for further downstream processing.
A batch process could be improved by repeatedly refreshing the culture medium without thereby removing cells. However, for this purpose, fresh medium must be added to the cell culture during the fermentation or alternatively a part of the culture medium must be repeatedly removed even though it has by no means been consumed. Such a process is expensive because especially mammalian cell culture medium is difficult to develop and manufacture and consequently is expensive. In this regard the so-called “feedbatch (alternatively named fed-batch) process” is a process in which, during the fermentation procedure, fresh culture medium is not supplied in its totality but only the consumed nutrients are supplied. However, in practice this process does not provide any substantial advantages due to an increase of the waste materials leading to a characteristic course of the cell density during the culturing procedure similar to that in the case of the purely batch process.
The third process is the continuous process. Here, the environmental conditions can be uniformly adjusted so that the cells can grow optimally. However, the process is very laborious and expensive because culture medium must be continuously supplied and removed (with cells and product). Furthermore, in the case of this process too, there is not achieved a substantially higher cell density than in the case of the above-mentioned processes because cells are also continuously removed from the reactor with the running off of cell culture medium.
An example of a special continuous process is the so-called perfusion process. In prior art perfusion culture methods, the waste/impurities in the medium is continuously removed (cells plus product is retained in the bioreactor) from the culture and the displaced medium is replenished with fresh medium. The constant addition of fresh medium and elimination of waste products provides the cells with the environment they require to achieve high cell concentrations and with that higher productivity. Thus, it is possible to achieve a state of equilibrium in which cell concentration and productivity are maintained. Product may be continuously harvested by taking out medium (with cells and product) or via a so-called bleed.
In summary, in a continuous process the bioreactor many times does not comprise a filter that can allow impurities to be removed while retaining cells and high molecular weight compounds (e.g. product) in the bioreactor. In a continuous perfusion process the bioreactor comprises one filter to remove impurities while retaining cells and product or one filter that only retain cell, i.e. both product and impurities pass the filter. Said in other words, the prior art bioreactors comprises only one filter.
U.S. Pat. No. 6,544,424 describes a filtration system for biological fluids creating an alternating tangential flow (ATF) of fluid through a filter element where waste fluids can be removed from the culture by filtration and fresh fluid may be added to replenish the filtered fluid. In the present context this may be seen as an example of a reactor with ONE filter to allow impurities to be removed while retaining cells and high molecular weight compounds (e.g. product) in the bioreactor.
In FIG. 1 herein a prior art reactor may be seen as a reactor with only the UF filter unit to remove impurities or only the filter to remove product and impurities, i.e. a reactor that does not comprise both filters shown in FIG. 1. In other words, the reactor described in e.g. U.S. Pat. No. 6,544,424 lacks the possibility of harvesting high molecular biological products from one filter (product filter) at an adequate out flow speed simultaneously with removing impurities from the culture vessel using a second filter (impurity filter) at an adequate out flow speed.
The following prior art documents:                EP270905A        Stark et al., Advances in biochemical Engineering/Biotechnology 2003 vol. 80, 149-175;        Linardos et al., Biotechnology and Bioengineering 1992, vol. 39, 504-510        Poertner et al., Applied Microbiology and Biotechnology, 1998 vol. 50, 403-414 essentially describes prior bioreactors and impurity removal systems as discussed above and illustrated in FIG. 1—i.e. prior art reactors that lack the possibility of harvesting high molecular biological products from one filter (product filter) at an adequate out flow speed simultaneously with removing impurities from the culture vessel using a second filter (impurity filter) at an adequate out flow speed.        